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By themselves, the video conferencing systems described in the previous article
allow you to hold a point-to-point -- one-on-one -- videoconference with any
other H.323 video conferencing user. To hold a multipoint video conference --
one involving three or more endpoints -- over the Internet you also need an
H.323 multipoint control unit (MCU) videoconference server and an H.323
Gatekeeper.
Figure 5: Point-to-point videoconference
A gatekeeper or MCU is not needed; one participant can simply "call"
the other, either by domain name or IP address.
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Figure 6: Multipoint videoconference
In a multipoint conference, each participant calls the designated MCU,
with their local Gatekeeper acting behind the scenes (between each endpoint
and the Internet and between the MCU and the endpoints) to aid and facilitate
the conferencing. As shown in the illustration, the MCU (or MCUs) running
a conference can be anywhere on the Internet.
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An MCU is a central multimedia server to which video conferencing terminals
connect. The MCU manages the audio and video for each participant, providing,
in a sense, a virtual meeting room for the group. For larger conferences, several
MCUs can be cascaded to increase capacity and for load balancing.
An H.323 Gatekeeper is another server; it controls and manages video conferencing
resources, including H.323 MCUs. It operates behind-the-scenes, confirming what
the various terminals can do, assisting in call setup and take-downs, address
translation, admission control, bandwidth management, zone and call management,
and so forth.
pIn an H.323 environment, all complaint video conferencing systems automatically
register themselves with their local zone's Gatekeeper. To further simplify
matters, Gatekeeper administrators often exchange Gatekeeper information, establishing
so-called neighboring Gatekeepers, as has been done between UIC and Urbana.
The UIC H.323 Gatekeeper and MCUs are run by the ITL. The UIC MCUs can handle
multipoint conferences at speeds from 128 Kbps to 1.5 Mbps and groups of three
to nine terminals, or any combination thereof, and also continuous
presence multipoint conferences, ones where the video screen is split
in four quadrants like the Brady Bunch. Contact Peter Garcia, (312)996-7144
or audio@uic.edu, to reserve them. If needed,
additional H.323 MCU capacity is available from the CIC.
Comments are welcome; please send them
to Ed Garay, garay@uic.edu
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